Fiber optic network equipment and test equipment often require careful control of the polarization of light propagating in a fiber optic cable. One method of obtaining such polarization control is to insert a series of wave plates in the light path of the propagating light. However, utilizing wave plates typically involves directing light out of the fiber and redirecting the polarization corrected light back into an optical fiber. Such techniques are complicated and require careful alignment. Redirecting the polarization corrected light back into the optical fiber also results in back reflections and insertion losses.
In order to control the polarization of light in a fiber without the losses and alignment problems associated with wave plate systems, several patents describe a technique to control the polarization of light propagating in a fiber by applying pressure to an optical fiber. These references include U.S. Pat. No. 4,988,169 entitled xe2x80x9cOptical Signal Control Method and Apparatusxe2x80x9d issued to Neigle G. Walker; U.S. Pat. No. 4,753,507 entitled xe2x80x9cPiezoelectric Loading Housing and Methodxe2x80x9d issued to Ramon P. DePaula et al; and U.S. Pat. No. 5,903,684 entitled xe2x80x9cIndependent Control of Normally Interdependent Light Transmission Characteristics of Optical Fiberxe2x80x9d issued to Robert M. Payton. All three patents are hereby incorporated by reference.
Each of the three references describes a polarization compensation system that utilizes a plurality of fiber squeezers. Each fiber squeezer squeezes a different segment of the optical fiber. It is known that applying a transverse compressive force to a length of optical fiber changes the refractive index of the fiber via a photoelastic effect and introduces a stress induced birefringence. By applying transverse pressure along different directions, each fiber squeezer rotates the polarization of light propagating in the optical fiber about orthogonal axes on a Poincare sphere.
Although the principles of using pressure on a fiber to control polarization are well documented, one problem with building such systems is high signal losses caused by fiber squeezing. Typical activation induced losses in such systems are in the 0.5 dB range. The activation-induced loss measures the addition insertion loss caused by the activation of the device and is defined as the difference of the maximum and minimum insertion loss of the device at all activation conditions. This specification is particularly important because all polarization-impairment compensation schemes involve a feedback signal to activate the polarization controller. The activation-induced loss causes errors in the feedback signal and directly degrades the performance of the compensation apparatus. When a polarization controller is used in an instrument for measuring the polarization dependent loss (PDL) of optical components, the activation-induced loss limits the resolution and accuracy of the measurement. Controller PDL also contributes to error in the feedback system for PDL measurements and complicates the design of compensation hardware and software.
Current pressure based polarization controllers also suffer reliability problems because the applied pressure causes fiber fracturing and breakage. For example, the DePaula reference (507 patent) states that at room temperature, fiber fracturing begins when the fiber is deformed by only 1 percent. For example, a 125 micrometer glass fiber begins fracturing when the deformation is only 1.25 micrometers. To control fiber breakage and minimize losses, Shimizu reference describes coating the fiber with metal prior to the application of pressure. However, uniform metal coatings are not easily reproducible in production.
Another problem with prior art fiber squeezing systems is that high voltages are needed to drive the piezoelectric actuators that move the squeezers. Thus the driver circuits of the piezoelectric actuators require large power supplies and transformers to xe2x80x9cstep upxe2x80x9d the voltages. These additional components increase the size and cost of the polarization controllers.
Thus an improved system for minimizing activation losses, minimizing fiber breakage and reducing the power needed to drive the piezoelectric is needed.
The present invention describes an improved polarization control system for controlling the polarization of light in an optical fiber. The systems uses various mechanisms such as piezoelectric drivers to control a fiber squeezing surface that applies a transverse compressive force to a segment of optical fiber. In one embodiment of the invention, the piezoelectric driver is driven at a resonance frequency to minimize the voltages needed to operate the piezoelectrics. The fiber-squeezing surface are preferably super polished to reduce irregularities below 100 microns. To minimize the possibility of optical fiber breakage, a poly-imide coating is maintained between a core of the fiber and the surface applying the transverse compressive force.